what kind of mattress doesn’t sag/dip?

so i’ve had this mattress for less than a year. my main concern was size, nothing less or more. i was buying a new frame and all as well so i just got a cheap mattress. it’s SUPER lightweight, i couldn’t tell you what it’s made of but what i can say is that in the ~10 months i have had it, the side i sleep on has a HUGE flat dip, so much so that i have to put towels underneath the mattress to lift it up. it’s like sleeping on a blanket not a mattress.

i’m 125 lbs so idk why it wore out and dipped like this so quickly. the other side of the mattress is still firm/full. i switch to that side sometimes but still i need a better mattress that won’t have this problem and i now can splurge a little if necessary.